Solution

The task text is multitap code for "ninehundredandfifty". Calling this number (hinted by multitap) on the phones previously visited results in a message prompting the user to dial "3" for the answer. It's not possible to detect dialed numbers on a rotary phone, so a DTMF tone must be played into the microphone from another device (such as a mobile phone). Doing so will give you the answer salary. The title hints that you should go back (to the telephones) to the future (future phone, aka DTMF tone).